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J ] o [0 I A~ Q.). (.1i. Q"J. . ; .1 il j {" ~ i :.::s :) r.~ ..:) ~J .:; n :, ~ ~~ " ~,.~ " '.~ ':.:;. .-':-. .."" .; . ~ ;:"\ ~ .J Paul Holden BIOIWEST, Inc. 1063 West 1400 North Logan, UT 84321 Dear Paul: 28 Sep. 1998 I have now had a chance to read the draft report on "Flow Recommendations for the San Juan River" (16 Sep 1998). 1 focused my attention on the portions of the report relating to hydrology and geomorphology. In general, I think that the report is well-organized and written, and represents a great dea:! of very thorough, careful work. My concerns following the last meeting of the review panel, in May 1998, were addressed in this document with the explanation of how bankfull discharge was detennined, and the list of habitat types designated for the San Juan River. I think that the process followed for developing flow recommendations on the San Juan River represents a model that can be usefully applied to other regulated rivers with endangered species concerns. I realize that the process of making flow recommendations for the San Juan River will involve on- going changes and adaptive management, as stressed throughout the flow recommendations report. My suggestions for continued future research related to geomorphology are (I) to develop a system-wide sediment budget, and (2) to use 2-dimensional flow and sediment transport modeling to study the effect of changing water and sediment discharge regimes on backwater habitats. At present, the conceptual models of how the San Juan River adjusts to changing flow regimes are black-box in that they are based on observed changes and historical information, rather than specific, quantitative knowledge of processes such as sediment movement and channel response. Even an approximate quantification of sediment inputs, storage, and outputs from the San Juan River corridor would help to explain long-term trends or different channel responses through time to the same discharge level, for example. A system-wide sediment budget could be used in a manner analogous to the RiverWare model in that it would be more feasible than at present to evaluate the system-wide effects of changes in one component (such as tributary sediment input). Similarly, 2d models which simulate the secondary flow patterns that are likely responsible for creating and maintaining backwater habitats could be used to more effectively evaluate the response of these habitats to changes in flow regime. In closing, I would like to reiterate that I am very impressed with the hydrologic and geomorphic studies conducted on the San Juan River in connection with developing flow recommendations for endangered species recovery. My only specific comments at this point are suggestions for possible future directions for the program. Sincerely, ~jQV/\_1 NiL Ellen Wohl Associate Professor of Geology Colorado Slate University Comments and Responses Flow Recommendations Draft Report Wohl-1